The need to develop alternative clean renewable energy supplies is becoming critical. Problems of climate change and global warming due to carbon emissions are becoming an accepted fact. The dramatic rise in the cost of petro-chemical energy is due to decreased supplies and increase in world demand. There is increasing political instability of regions of the world that have large oil supplies. There are geo-political and economic ramifications of being dependent on imported energy or on being economically dependent on the export of energy from dwindling energy reserves.
Heretofore, space solar power has been considered as an alternative clean energy source. Space solar power is clean, inexhaustible, available 24 hours a day, and has the potential to generate as much energy as terrestrial power plants. However, because of the filtering effect of the Earth's atmospheric gases (air acts as an insulator), and for other reasons such as reflection and absorption, only a fraction of solar energy in any given area reaches Earth. The average solar power per unit area outside Earth's atmosphere during any given time period is about 136% that available on Earth's surface during direct sunlight (1336 W/m2). Various apparatus and methods have been considered, and some developed, to compensate for this shortcoming.
A very recent proposed solution is described in pending U.S. Patent Application No. 2009/0171477 of Neyfeh et al. which describes systems and methods that employ high-intensity lasers to set up a thin plasma sheet, also called a waveguide or “hot shell”, in the atmosphere as a function of beam intensity and geometry. A laser beam can be spread and directed with physical optics to generate a thin inverted cone-shaped hot shell waveguide in the atmosphere. According to Neyfeh, the hot shell of the waveguide has a different index of refraction from that of the surrounding air layers and as such serves to internally reflect portions of the entering solar ray entering an aperture in the hot shell, toward the tip of the cone and a solar energy storage component positioned there, thus providing a virtual solar energy concentration system. The Neyfeh et al. systems and methods, therefore, endeavor to make the best of the filtered solar energy present in Earth's atmosphere.
Another proposed solution is to collect the Sun's energy in space, where it is more concentrated, via satellite-based or lunar-based collection stations. The energy must then be transmitted from the collection point in space to the Earth's surface. Since wires extending from Earth's surface to an orbiting satellite would be impractical, many space-based solar power designs have proposed the use of microwave or laser beams for wireless power transmission. The collecting station would convert solar energy into electrical energy, which would then be used to power a microwave emitter or laser directed at a collector on the Earth's surface. There are numerous technical, political, legal and economic challenges to building space solar power stations. Limitations in photovoltaic technology and the difficulty of building large structures in space are some of the issues. In the past, launch costs have been prohibitive. In addition to the relatively high costs involved with this method, other problems include cumulative radiation damage or micrometeoroid impacts.
The underlying physics of wireless power transmission resembles that of wireless communications, but with an important difference. Unlike information transfer, where the percentage of received power must be only sufficiently high enough to recover the signal, wireless power transmission places a critical emphasis on the maximum amount of possible energy transfer and conversion efficiency. Ideally, a wireless power transmission system would have the ability to transmit any amount of power to any point in space, but practical limitations such as conversion efficiencies at the source and the receiver, and disturbances in the transmission medium will always limit the performance of an implemented system.
For instance, as recently as 2006, the highest amount of energy obtained from a receiver when testing high intensity lasers to illuminate vertical multi-junction solar cells developed by NASA-GRC was 23.7778 watts. One vertical multi-junction solar cell was able to achieve a power density of 13.6 watts per cm2, at a conversion efficiency of 24%. Although these experiments confirm that the VMJ technology can withstand and utilize the high intensity laser energy without damage or significant reduction in previously known conversion efficiencies, wireless power transmission via high intensity laser beaming technology is not yet ready for high order energy transfer. In short, the main limiting factors to the laser power beaming system are the conversion efficiencies of the laser (electrical to photonic) and the photovoltaic cells (photonic to electrical). Although there are a variety of photovoltaic cells on the market approaching conversion efficiencies of 40% (such as triple junction cells), these technologies cannot operate at intensities 1000 times or greater than that of the sun.
Based on the above, it is clear that there is a strong desire to harness the energy existing in both our atmosphere and beyond, but it is apparent that the various methods and apparatus designed to capture and utilize this energy source are attendant with various, problems, shortcomings and limitations.